When producing hydrocarbons from a wellbore, it is often advantageous to inject chemicals such as treatment fluids into the production fluid. This may, for example, help to prevent the deposition of sulphur from or change the viscosity of the production fluid.
In wellbores intended for the production of natural gas, a degree of liquid—this may be water or hydrocarbons—is often also produced. In the early stages of production, the reservoir pressure is often sufficient to remove these liquids from the wellbore. As production continues, the reservoir pressure drops and often becomes insufficient to remove liquids from the well which may accumulate in the wellbore and inhibit the production of natural gas. In this case, injection of a foaming agent may assist to reduce the density of liquids which have accumulated in the wellbore. This may facilitate their removal from the wellbore at lower reservoir pressures.
It is known in the art to use capillary injection strings inserted into the wellbores to inject treatment fluids into the wellbore. Such capillary injection strings often have a very small diameter and may include several components, such as valve assemblies. There may be some applications which require low flow rates, and which require a valve in the valve assembly to provide a very small open area. Such a small open area may increase the liability of this type of valve to blocking by unwanted particles in the fluid. The small diameter of the components increases their likelihood of becoming blocked. This may therefore introduce complexities of frequent replacement of components within the capillary string.